I was installing 3ds max 9 from the FIRSTbase site, and after it finished installing, I exited out of the window that had the activation key thing on it.
I THOUGHT I had it copied in my clipboard, but when I pasted it, it was something else I had over copied.
Just go back to the page when you first log into the website. Then click the thing that says “re download 3Ds max”, if you go through the process again it will show you the key again. At least it worked for me with inventor.